In rotary wing aircraft, such as helicopters, two or more elongated rotor blades are mounted on a central rotor hub. The blades normally extend generally radially from the hub and are rotatable with the hub about its central axis. When the rotor hub is being driven, centrifugal, aerodynamic, and other forces are exerted on the rotor blades and tend to cause the blades to pivot relative to the hub. The pivoting generally occurs in a plane in which the blades normally lie and which is typically oriented normal to the central axis or axis of rotation of the rotor hub. In-plane pivoting motion of a rotor blade, which is also termed lead-lag or drag motion, represents an effort by the blade to respond to the forces acting upon it and to seek a position of equilibrium. The equilibrium position of each rotor blade constantly changes, however. The primary cause of changes in the in-plane equilibrium position of a rotor blade is Coriolis moments produced by vertical flapping motions of the blade. Other causes include changes in the aerodynamic forces on the blade due to crossflows of air over the blade. The lead-lag motion that results from such changes in the blade's equilibrium position appears as a swinging motion of small amplitude with respect to the rotor hub.
One method of mounting the blades of a helicopter rotor on the rotor hub is to provide a blade retention system between each rotor blade and the hub which is relatively stiff or inflexible with respect to in-plane or lead-lag motions of the rotor blade. Typically, a stiff in-plane blade retention system consists of a relatively inflexible or rigid interconnection between a rotor blade and the rotor hub. Thus, for example, the rotor blade may be bolted directly into a corresponding socket formed in the rotor hub. The result of utilizing such a stiff in-plane blade retention system is that the rotor blade has relatively little freedom of movement in the lead-lag direction and the natural frequency of oscillation of the rotor blade in the lead-lag mode is greater than the nominal rotational speed of the rotor hub and blades. Stated another way, a stiff in-plane blade retention system is, by definition, a system in which the rotor blade completes more than one full cycle of lead-lag motion per revolution of the rotor hub at the natural frequency of lead-lag motion. The advantage of such a relatively high natural frequency for the lead-lag motion of a rotor blade is that destructive ground resonances, which are discussed below, can be avoided. The disadvantage of the high natural frequency of a stiff in-plane blade retention system is that very large loads and bending moments are imposed directly on and must be borne by the rotor blade and hub. The rotor is forced to accommodate oscillatory lead-lag moments which are still imposed on the rotor and which cause significant fatiguing of the materials of which the rotor blades and hub are constructed. The problem of fatigue failure in rotors having relatively stiff in-plane blade retention systems is presently such that stiff in-plane retention systems are not typically used. Stiff in-plane systems probably will not be used commercially unless and until some significant progress is made in developing new materials for such systems.
The other method of mounting the blades of a helicopter rotor on the rotor hub is to provide an articulated blade retention system between each rotor blade and the hub. Although all such articulated blade retention systems may be described as relatively soft in-plane, with reference to lead-lag motions of the rotor blades, articulated blade retention systems are typically classified as fully articulated, on the one hand, and soft in-plane, on the other hand. Fully articulated blade retention systems produce a natural oscillation frequency of a rotor blade in the lead-lag mode of about 0.25 to 0.35 cycles per revolution of the rotor hub. In comparison, soft in-plane blade retention systems produce a natural oscallation frequency of a rotor blade in the lead-lag mode of about 0.65 cycles per revolution of the rotor hub. Because both types of articulated blade retention systems afford the rotor blades additional freedom of movement in their plane of rotation, any rotor which incorporates such a system has a potential condition of instability at at least one critical speed of rotation while the helicopter is resting on the ground. The instability, which is termed ground resonance, is produced by a coupling between the motion of the supporting structure for the rotor hub, including the landing gear and tires, and the pendular motions of the blades as they pivot in the lead-lag direction. Due to the elastic flexibility of the supporting structure and the pendular frequency of the blades, the coupling action may produce, at the critical speed(s), violent and uncontrollable rotor blade oscillations that may reach destructive proportions if uncontrolled.
Articulated blade retention systems are the primary blade retention systems used in helicopters at present because the hinge mechanisms that provide the articulation also tend to avoid the structural stresses and materials fatigue failures that are encountered in stiff in-plane rotor blade retention systems. The undesirable ground resonances that are encountered when using articulated blade retention systems are usually accommodated and effectively mitigated by utilizing auxiliary damping devices. The damping devices are effective at the critical frequencies of ground resonance (i.e., the natural frequencies of lead-lag motion) to damp and reduce the resonant motions of the rotor blades. Although the use of auxiliary lead-lag dampers introduces additional weight, cost and complexity into a helicopter rotor, the disadvantages have been accepted for the present because of the insurmountable materials problems presented by relatively stiff in-plane rotor blade retention systems.
Early proposals for articulated rotor blade retention systems incorporated antifriction bearings, such as roller bearings or ball bearings, to provide the necessary articulation. More recently, articulated retention systems have been developed which incorporate one or more laminated elastomeric bearings, rather than antifriction bearings. Such laminated bearings comprise a plurality of alternating and bonded together layers of elastomeric material and substantially inextensible material. The bearings do not require lubrication, have only limited maintenance requirements, and affords a reduction in weight as compared to antifriction bearings. The advantages of laminated elastomeric bearings are such that the bearings have found widespread use and acceptance throughout the helicopter industry and have been incorporated in a number of different helicopter rotor blade retention systems.
Blade retention systems that incorporate laminated elastomeric bearings typically include auxiliary lead-lag dampers. Examples of such systems are described and illustrated in Mosinskis U.S. Pat. Nos. 3,501,250, Rybicki 3,759,631, and Rybicki et al 3,764,230. Since elastomers possess some inherent internal damping capabilities, some auxiliary lead-lag dampers, such as the one shown in Potter U.S. Pat. No. 3,842,945, utilize elastomer to provide the necessary damping. It has also been suggested, however, that the laminated elastomeric bearings of articulated blade retention systems might themselves provide all of the lead-lag damping that is required in a helicopter rotor. Bearings that function as dampers would eliminate the extra weight, cost, and complexity of auxiliary lead-lag dampers. Thus, for example, Gorndt et al U.S. Pat. No. 3,111,172 suggests, at column 2, lines 38-41, that elastomeric bearings may eliminate the need for an auxiliary damper because the elastomeric bodies in the bearings can be selected to have an internal friction sufficient to provide the necessary damping.
Although the Gorndt et al patent proposes to eliminate an auxiliary lead-lag damper, the blade retention system that is described and illustrated in the patent is not capable of achieving such a desirable result in practice. The difficulty with the Gorndt et al blade retention system is that a single, main laminated elastomeric bearing is utilized to accommodate not only lead-lag motions of an attached helicopter rotor blade, but also flapping motions of the blade. Flapping motions are oscillatory movements of the blade which are similar to lead-lag motions, but which occur in planes generally perpendicular to the plane of rotation of the blade and to the plane in which lead-lag motions occur. In a typical helicopter rotor, the normal flap motion of each rotor blade is seven to eight times the lead-lag motion (e.g., + or - 3.5.degree. in flap versus + or - 0.4.degree. in lead-lag). In order to accommodate the flapping motions of a rotor blade, the main laminated bearing of the Gorndt et al blade retention system must incorporate a relatively large total thickness of elastomer. The large thickness of elastomer necessarily reduces the spring rate of the bearing in the lead-lag direction and the natural frequency of oscillation of the rotor blade about the lead-lag axis. As the lead-lag spring rate decreses, the degree of damping (i.e., the loss factor) that must be provided at the natural frequency of lead-lag motion increases to a point where no presently available natural or synthetic elastomer can produce the required damping. Thus, in practice, damping for the Gorndt et al blade retention system must be provided by an auxiliary damper, rather than by the elastomeric laminations of the laminated bearing.